Where would one put general requirements for plasmoids? For example I think that a plasmoid should be able to glue itself to a normal window, e.g. a kopete chat-window and act as "plug-on", i.e. communicate with the chat-window and e.g. display the buddy-icon of that chat-partner or contact-details, such as new email received by kmail etc.
So plasmoids would have to be able to start automatically when a new chat-window opens and glue themselves to it.